There is increasing evidence to suggest that HTLV-II infection may be more prevalent in this country than previously believed. Current licensed assays are designed to screen for HTLV-I, the other known member in the family of Human T-cell leukemia viruses. Therefore, HTLV-II antibodies are presently detected only through crossreaction. Further study is required to determine whether an HTLV-II specific assay should be introduced to directly detect HTLV-II antibodies. This will be achieved through developing recombinant protein-based assays for screening HTLV-II antibodies, and evaluating whether the developed assays detect more seropositive individuals than the licensed HTLV-I screening assays. In order to learn more about the pathogenic potential of HTLV-II, an effective serological means for differentiating populations infected by these two viruses will be desirable. This can be achieved by mapping the type-specific antigenic epitopes in regions of the virus proteins where the amino acid sequences of HTLV-I and HTLV-II are most divergent. A central issue concerning the screening of the blood supply is the question of whether some as-yet-unidentified members in this family of human retroviruses exist. Such viruses may be linked to neurological disorders such as multiple sclerosis or may be present in high-risk populations which may pose a threat to our blood supply. The proposed approach is to identify epitopes in the pol region that are shared by HTLV- I and HTLV-II and to determine whether the presence of pol antibodies directed to the conserved epitopes can be used as a marker of infection by an member in the family of Human T-cell leukemia viruses.